Multicentricity and bilaterality are well-established characteristics of breast carcinoma, but little is known about the relationship of these variables with each other. This question was explored by analyzing the data pertaining to 880 women with invasive breast carcinoma. Patients with multicentric carcinoma had bilateral disease more often than those whose carcinoma was apparently limited to a single quadrant. Among women who had lobular carcinoma in situ coexisting with infiltrating duct carcinoma or infiltrating lobular carcinoma, bilaterality and multicentricity were significantly more common than they were among patients whose only lesion was infiltrating duct or medullary cancer. Other variables associated with bilaterality and multicentricity were degree of ductal differentiation, tumor size, nodal status, type of tumor margin, intensity of lymphoid infiltrate, and menstrual status. Age at diagnosis and estrogen receptors were related to bilaterality but not to multicentricity. The following variables proved to be unrelated to bilaterality and multicentricity: family history of breast carcinoma, height, weight, and parity. The data obtained in this study tend to support a conclusion that multicentricity and bilaterality are manifestations of similar factors involved in the neoplastic transformation of mammary gland epithelium leading to the development of breast cancer.